Alkanethiol-Induced Structural Rearrangements in Silica-Gold Core-Shell-type Nanoparticle Clusters: An Opportunity for Chemical Sensor Engineering Frank Osterloh,* Hiroki Hiramatsu, Rhiannon Porter, and Ting Guo* Department of Chemistry, University of California, One Shields Avenue, Davis, California 95616 Received May 21, 2003. In Final Form: April 19, 2004 Electrostatically bonded SiO2‚Au nanoparticle clusters form by reaction of 3-aminopropylsilane-modified SiO2 spheres (470 nm) with citrate-coated gold nanoparticles (9.7 nm) in water. Reaction of the clusters with 0.01 M KBr or HCl solution induces desorption of the gold nanoparticles within minutes. Reaction of the clusters with alkanethiols CnH2n+1SH (n ) 2-18) at 80 °C causes the gold nanoparticles to form stringlike gold nanoparticle structures for thiols with short alkane groups (n ) 2, 3, 4) and hexagonally packed arrays of gold nanoparticles for thiols with long alkane groups (n ) 5-18) on the silica surfaces. The structural changes indicate that the bonding between Au and SiO2 nanoparticles has changed from electrostatic to van der Waals. Elemental analyses show that the reaction with hexanethiol does not affect the Au/Si/O composition of the SiO2‚Au cluster, and Raman spectra on the hexanethiol-reacted cluster indicate the formation of a thiol SAM on the gold nanoparticles. The thiol-reacted SiO2‚Au clusters display characteristic shifts of the absorption maxima in the visible spectra, and there is an inverse relation between these shifts and the lengths of the alkyl groups in the thiols. This relationship can be understood in terms of the free electron model for metals. The use of SiO2‚Au nanoparticle clusters as coulometric sensors for the qualitative detection of thiols is discussed. Introduction As nanoparticle-based bottom-up assembly strategies to materials and devices continue to evolve, 1-7 it becomes increasingly important to understand and control the bonding interactions between inorganic nanoparticles. The nature of the nanoparticle-nanoparticle bond profoundly influences the collective chemical and physical properties of nanoparticle-based materials. The bonding determines the stability of the aggregate, its reactivity with small molecules and other nanoparticles, 8 and the distance between the bonded nanoparticles, which determines the electronic and magnetic coupling between the nanopar- ticles. Core-shell-type SiO 2 ‚Au clusters of the general struc- ture shown in Figure 1 provide a convenient medium to study the influence of the bonding on the properties of a nanoparticle aggregate. These clusters consist of Au nanoparticles that are attached to the surfaces of sub- micrometer SiO 2 spheres with organic linkers. Groups with various linkage capabilities can be readily introduced to the surface of the silica particles by means of organosilanes 9-11 and to the gold nanoparticles by means of thiols. 12-15 Appropriate modifications and reactions of the functionalized nanoparticles have allowed the syn- thesis of a variety of electrostatically and covalently bonded SiO 2 ‚Au clusters (see Figure 1 and Table 1), 8,16-19 some of which have been proposed for electromagnetic signal processing applications 20-22 and as biosensors. 19 We report here on the electrostatic assembly of 3-ami- nopropylsilane-terminated 470-nm silica spheres and citrate-coated gold (9.7 nm) nanoparticles and on the reactions of the resulting SiO 2 ‚Au core-shell-type ag- gregates with electrolytes and with various linear chain alkanethiols. As we will show, alkanethiols can cause structural rearrangements of the SiO 2 ‚Au clusters, which are accompanied with characteristic changes of their optical spectra. 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